Transmit beamforming is an energy-efficient wireless communication technique that allows a transmitter with two or more antennas to focus its bandpass signal in an intended direction. The idea of transmit beamforming has been extended to networks of single-antenna cooperative transmitters that pool their antenna resources and behave as a ldquodistributed beamformer.rdquo Unlike conventional transmit beamforming, however, the carriers of the single-antenna transmitters in a distributed beamformer are each synthesized from independent and imperfect local oscillators; carrier phase and frequency synchronization among the transmitters is necessary to ensure that a beam is aimed in the desired direction. In this paper, a new time-slotted round-trip carrier synchronization protocol is proposed to enable distributed beamforming in multiuser wireless communication systems. Numerical results for a distributed beamformer with two transmitters show that the parameters of the round-trip synchronization protocol can be selected such that a desired level of phase accuracy and reliability is achieved during beamforming and such that the synchronization overhead is small with respect to the amount of time that reliable beamforming is achieved. The impact of mobility on the performance of the round-trip carrier synchronization protocol is also shown to be small when the synchronization timeslots are short.
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